Wall-flow honeycomb filters are used to remove solid particulates from fluids, such as in exhaust gas streams. FIG. 1 illustrates a typical prior art wall-flow honeycomb filter 100. The honeycomb filter 100 has an inlet end face 102, an outlet end face 104, and an array of interconnecting porous walls 106 extending longitudinally from the inlet end face 102 to the outlet end face 104. The interconnecting porous walls 106 define a grid of inlet cells 108 and outlet cells 110. The outlet cells 110 are closed with plugs 112 where they adjoin the inlet end face 102 and open where they adjoin the outlet end face 104. Oppositely, the inlet cells 108 are closed with plugs (not shown) where they adjoin the outlet end face 104 and open where they adjoin the inlet end face 102. Such filters 100 are typically contained in a rigid housing (not shown). Fluid directed at the inlet end face 102 of the honeycomb filter 100 enters the inlet cells 108, flows through the interconnecting porous walls 106 and into the outlet cells 110, and exits the honeycomb filter 100 at the outlet end face 104.
In a typical cell structure, each inlet cell 108 is bordered on one or more sides by outlet cells 110, and vice versa. The inlet and outlet cells 108, 110 may have a square cross-section as shown in FIG. 1 or may have other cell geometry, e.g., rectangle, triangle, hexagon, octagon, etc. Diesel particulate filters are typically made of ceramic materials, such as cordierite, aluminum titanate, or silicon carbide. For diesel particulate filtration, honeycomb filters having cellular densities between about 10 and 300 cells/in2 (about 1.5 to 46.5 cells/cm2), more typically between about 100 and 200 cells/in2 (about 15.5 to 31 cells/cm2), are considered useful in providing sufficient thin wall surface area in a compact structure. Wall thicknesses can vary upwards from the minimum dimension providing structural integrity of about 0.002 in. (about 0.05 mm), but are generally less than about 0.060 in. (1.5 mm) to minimize filter volume. A range of between about 0.010 and 0.030 in (about 0.25 and 0.76 mm) is most often selected for ceramic materials such as cordierite, aluminum titanate, and silicon carbide at the preferred cellular densities.
When particulates, such as soot found in exhaust gas, flow through the interconnecting porous walls 106 of the honeycomb filter 100, a portion of the particulates in the fluid flow stream is retained on or in the interconnecting porous walls 106. The efficiency of the honeycomb filter 100 is related to the effectiveness of the interconnecting porous walls 106 in filtering the particulates from the fluid. Filtration efficiencies up to, or in excess of, 90% by weight of the particulates can be achieved with honeycomb filters having properties such as described above. However, filtration efficiency or integrity of a honeycomb filter can be compromised by manufacturing defects such as holes, cracks, or fissures. Such defects allow the fluid to pass through the filter without proper filtration. Thus, in the production of honeycomb filters for applications such as diesel particulate filtration, it may be desirable to test the honeycomb filters for the presence of such defects that may affect filtration efficiency. Honeycombs with detected defects may be repaired, or if irreparable, discarded.
U.S. Patent Application Publication No. 2003/0112437 (Enomoto et al.) discloses a method of detecting defects in a diesel particulate filter using a particulate, such as smoke. The method involves generating particulates and directing them at an inlet end face of the filter such that the particulates enter the filter. Cells having defects readily allow the particulates inside them to flow into the adjacent cells or through the defective plugs. Thus, numerous, typically larger, particulates emerge at the outlet end face of the honeycomb filter from cells/plugs having defects. A light source, such as a laser source, is positioned to emit light such that the light passes in the vicinity of the filter to irradiate the particulates emerging therefrom. A camera is installed above the filter to photograph reflected beams generated by particulates intersecting the light. Brighter spots in the photographed image correspond to cells/plugs containing defects.
Enomoto et al. discloses, in FIG. 1 thereof, a particulate inlet 6 for providing particulates to the inlet end face of the filter 20. The particulate inlet 6 is a pipe which has the same dimension as the filter and is axially aligned with the filter. When the particulate flows through the pipe, the flow of the particulate fluid near the wall of the pipe may be retarded relative to the flow of the particulate fluid at the center of the pipe due to fluid flow phenomena, thereby possibly leading to cells closer to the periphery of the honeycomb filter receiving less particulates than cells farther away from the periphery of the filter. During testing, defective cells that offer lower resistance to flow will allow more particulates per unit time and larger particulates to pass through them, thereby providing an indication (as between the laser light and the particulates). If a defective cell is starved of flow, and, thus, particulates at the inlet end face, the brightness of the spot indication produced by such a defective cell would be weakened relative to the cells not so starved. Accordingly, such cells would not indicate a defect (or indicate it less dramatically) because of being starved of particulates, even though it is a defective cell.
From the foregoing, there is a desire to avoid ambiguity in test results, particularly at or near the periphery of the tested honeycomb filter.